One of the most important tasks for modern hearing aids is to provide improvement in speech intelligibility in the presence of noise. For this purpose, beamforming, especially adaptive beamforming, has been widely used in order to suppress interfering noise. Traditionally, the user of a hearing aid is given the possibility of changing between a directional and a omni-directional mode in the hearing aid (e.g. the user simply changes processing modes by flipping a toggle switch or pushing a button on the hearing aid to put the device in the preferred mode according to the listening conditions encountered in a specific environment). Recently, even automatic switching procedures for switching between directional and omni-directional modes have been employed in hearing aids.
Both omni-directional and directional processing offer benefits relative the other mode, depending upon the specific listening situation. For relatively quiet listening situations, omni-directional processing is typically preferred over the directional mode. This is due to the fact that in situations, where any background noise present is fairly low in amplitude, the omni-directional mode should provide a greater access to the full range of sounds in the surrounding environment, which may provide a greater feeling of “connectedness” to the environment, i.e. being connected to the outside world. The general preference for omni-directional processing when the signal source is to the side or behind the listener is predictable. By providing greater access to sound sources that the listener is not currently facing, omni-directional processing will improve recognition for speech signals arriving from these locations (e.g., in a restaurant where the server speaks from behind or from the side of the listener). This benefit of omni-directional processing for target signals arriving from locations other than in front of the listener will be present in both quiet and noisy listening situations. For noisy listening conditions where the listener is facing the signal source (e.g., the talker of interest), the increased SNR provided by directional processing for signals coming from the front is likely to make directional processing preferred. Each of the listening conditions just mentioned (in quiet, in noise with the hearing aid user facing or not facing the talker) occur frequently in the everyday experience of hearing-impaired listeners. Thus, hearing aid users regularly encounter listening situations where directional processing will be preferable to the omnidirectional mode, and vice versa.
A problem with the approach of manual switching between omni-directional and directional modes of the hearing aid is that listeners may not be aware that a change in mode could be beneficial in a given listening situation if they do not actively switch modes. In addition, the most appropriate processing mode can change fairly frequently in some listening environments and the listener may be unable to conveniently switch modes manually to handle such dynamic listening conditions. Finally, many listeners may find manual switching and active comparison of the two modes burdensome and inconvenient. As a result, they may leave their devices in a default omni-directional mode permanently.
However, whether directional microphones are chosen manually by the listener or automatically by the hearing instrument, directional processing is performed by a lossy coding of the sound. Basically directional processing consists of spatial filtering where one sound source is enhanced (usually from 0 degrees) and all other sound sources are attenuated. Consequently, the spatial cues are destroyed. Once this information is removed, it is no longer available or retrievable by the hearing aid or the listener. Thus, one of the major problems with such methods of manual or automatic switching between directional and omni-directional modes is the elimination of information, which occurs when the hearing instrument is switched to a directional mode, which may be important to the listener.
Though the purpose of a directional mode is to provide a better signal-to-noise ratio for the signal of interest, the decision of what is the signal of interest is ultimately the listener's choice and cannot be decided upon by the hearing instrument. As the signal of interest is assumed to occur in the look direction of the listener any signal that occurs outside the look direction of the listener can and will be eliminated by the directional processing. This is in compliance with clinical experience, which suggests that automatic switching algorithms currently being marketed are not achieving wide acceptance. Patients generally prefer to switch modes manually rather than to rely on the decisions of these algorithms.